The Question:

Does sugar affect the freezing of water? Why?

The Answer:

The presence of sugar (or salt, or any other dissolved substance
in water) does indeed lower the freezing point of water. This
phenomenon, called freezing point depression,
occurs because the particles of the dissolved substance interfere with
the formation of the crystalline bonds between water molecules that
give ice its solidity.

Interestingly, the addition of a solute (dissolved substance) to
water not only lowers the freezing point but also raises its boiling
point — a phenomenon referred to as boiling point
elevation. It does this by lowering the vapor pressure of water
(a tendency that describes a substance's tendency to turn into a
gas).

The vapor pressure, boiling point, and freezing point of water
(or a solution containing water and another substance) are referred to
as colligative properties — properties that
depend on the amount of the solute in a solution. For more information
on colligative properties, check out their encyclopedia
entry.